Peripheral Arterial Disease (PAD) is an international epidemic of a significant size. For example, in the United States, an estimated 12-20 million people are affected. Closely related to diabetes mellitus, PAD is the peripheral vascular component of the systemic disease atherosclerosis which also causes heart attack and stroke. The incidence of PAD is increasing rapidly. Once PAD becomes critical limb ischemia, the 5-year mortality is worse than cancer, heart attack, and stroke, with 69% mortality as illustrated in FIG. 1. People with coronary heart disease and/or carotid disease who also have PAD have 3 to 6 times the risk of a coronary event or stroke as those without PAD.
PAD is called a multi-vessel disease when it is in the peripheral arteries of the legs, the carotid arteries of the neck, and/or the coronary arteries of the heart. Because 90% of people with PAD have no symptoms and are unaware that they have the disease, it often goes unnoticed until they have either a cardiac event or cerebrovascular accident (CVA), or if it progresses until there is critical limb ischemia with severe leg pain on exertion, which can lead to amputation. Due to the “invisibility” of PAD, it is difficult to accurately determine how many people have PAD.
The American Heart Association, the American College of Cardiology, the American Diabetes Association, the American College of Physicians, and others, have generated a series of PAD-related guidelines that were revised in January 2011. They believe it is imperative that primary care physicians perform screening for PAD, especially with diabetic patients. Because of the increasing incidence and lower age of people developing PAD, they also reduced the recommended screening age.
A typical patient who should be screened for PAD often presents with a cluster of diseases such as diabetes, hypertension, dyslipidemia, coronary artery disease (CAD), and CVA or stroke. Only about 10% of all patients with PAD are symptomatic.
The definitive screening exam for PAD is an Ankle Brachial Index or ABI, and for patients with calcified vessels in their legs, a Toe Brachial Index or TBI. These exams have traditionally been performed by vascular specialists and vascular surgeons using a continuous wave (CW), bi-directional (zero-crossing) Doppler. The ABI is a non-invasive test that compares the highest systolic brachial pressure of the two arms to the highest ankle pressure in each leg by dividing the ankle pressure by the brachial pressure. The resulting number is the Ankle Brachial Index. A person with two legs has two ABI readings. A normal ABI is 1.0. A number below 0.99 shows the presence of decreased arterial blood flow due to PAD, with the disease severity increasing as the index decreases. Occasionally a diabetic patient will have very stiff vessels that are incompressible due to calcification of the arterial walls, and he will have an ABI greater than 1.3. To determine perfusion to the foot, these patients should also have a TBI performed, since vessels of the toe do not normally calcify like those of the legs.
Presently, a CW, bi-directional Doppler is recognized as the appropriate instrument for performing ABI measurements. ABI measurements taken using a CW Doppler are reimbursable by the Centers for Medicare and Medicaid Services (CMS). In the United States, Doppler is currently recognized as the gold standard for performing the ABI.
It is important in clinical practice to measure blood pressures for the ABI or TBI using the same technology and technique on the patient so that the pressures obtained from multiple locations on the patient can be directly compared. Other techniques of measuring blood pressure in the brachial arteries are also used; however, they have been shown to be unreliable and inaccurate when applied to the ankle arteries.
Most primary care physicians, unlike the vascular specialists currently performing ABI exams, have little or no recent experience using a Doppler and may feel uncomfortable relying on their own expertise to perform, analyze, and diagnose disease using this instrument. They are wary of sending a false-positive patient to a vascular surgeon and experiencing the embarrassment when, upon ultrasound scanning performed by the vascular surgeon, the patient is told there is no disease. They are also wary of the reverse when a false-negative results in missing a diseased limb that needs follow-up.
Through experience, vascular clinicians are able to find the best location over an artery where a clean waveform can be achieved with excellent Doppler sound, and by inflating a vascular cuff, obtain an accurate systolic pressure measurement. This involves specialized technique, applying the right size cuff, holding the Doppler sensor at the correct angle in regard to the arterial walls and subsequent blood flow, recognizing what is heard, understanding how to inflate and deflate the cuff correctly, and finally calculating the index for each leg.
As mentioned above, ABI and TBI examinations are reimbursable in the United States provided the procedure meets CMS requirements. If a resultant ABI is abnormally high (>1.3), then toe pressures are substituted for the ankle pressures since, unlike the legs, the toes do not calcify and are compressible. The pressures in the toes are measured by putting a pressure cuff on the toe and obtaining a photoplethysmograph waveform (PPG) distally on the pad of that toe. The resultant TBI reflects an accurate state of the arteries in the patient's legs. PPG is the standard instrument used to determine toe pressures in the United States because it is more accurate and easier to use than a CW Doppler. It is most difficult to perform a toe pressure using a Doppler since there is no major artery near the surface of the pad on the bottom of the toe, whereas the PPG transducer can be attached to the pad of the toe with Velcro, a clip, or tape, and can detect blood flow via the pulsating arterioles.
As with Doppler, photoplethysmography is not commonly practiced in primary care physician offices, and as such, the clinicians are typically not familiar with normal waveforms and the associated readings.
Providing a physician or other medical clinician with an easy-to-use, accurate, and reliable means of performing an ABI and/or TBI, either singularly or in conjunction with a bi-directional Doppler, can provide reassurance that the results are accurate, especially when the actual measurements and calculations are performed automatically and are verifiable.